I. Field of the Invention
This invention relates generally to a process for sorting particles. In particular, the present invention relates to a process for qualifying an automated system for inspecting and sorting particles by introducing a predetermined number of seed particles into the process stream. This invention also relates to the production and qualification of seed particles having at least one property whose value or range of values is the same as or about the same as a value or range of values of a corresponding property of known undesirable particles.
II. Background of the Related Art
Conventional mining and/or metal processing operations can involve an intermediate stage wherein fragments or particles of the desired ore or metal are transported along a moving surface for visual inspection and the removal of undesired impurities. Removal of impurity fragments or particles is important for applications in which tight elemental concentration tolerances are necessary to ensure that the material properties of the finished product are suitable for their intended use. In many cases, such impurities will have an appearance that is clearly distinct from standard ore or metal particles having the targeted composition, thereby enabling their visual identification and removal by human operators.
Titanium (Ti) is an example of a material which, for applications involving highly stressed components, requires visual inspection and sorting during production. Commercial processes, which are used to extract Ti from TiO2 and/or TiCl4, produce a sponge-like material known as Ti sponge which is then consolidated into the desired shape by remelting. Under certain conditions, the production of Ti sponge results in the formation of sponge particles which have been “burned” and subsequently converted to titanium oxide or titanium nitride. The incorporation of nitrided Ti sponge particles during remelting is undesirable because if they survive the melting phase their presence in the finished metal, alloy or manufactured product can lead to the formation of hard alpha material or low density inclusions. These inclusions, if undetected by various quality checks, can impact the effectiveness of the manufactured article. Nitrided or oxidized Ti sponge particles have an appearance which is distinctly different from that of normal Ti sponge, being a darker shade of color which is readily discernible to the naked eye. This distinction permits the identification of the undesired particles and their removal from the process stream by a human operator.
Current industry standards generally require that the Ti sponge used to fabricate certain end use components be subject to 100% visual inspection. However, the use of individual operators to inspect and sort the process stream can be a time consuming, labor-intensive and costly process since the moving surface typically must move slowly to facilitate inspection and removal of undesired particles by human operators.
Recent approaches to modernizing sorting processes have involved the implementation of automated sorting systems which are capable of automatically inspecting and sorting particles in a process stream. An example of such a system is set forth in U.S. Pat. No. 6,043,445 to Gigliotti, et al. (hereinafter “Gigliotti”) which is directed to an apparatus for color-based sorting of titanium sponge particles. In one embodiment, Gigliotti discloses the use of an imaging device to capture a color image of the product as it is transported on a moving surface. The image is converted to a color signal and is sent to a central processing unit where the signal is transformed to a color value. This color value is then compared to a look-up table which defines acceptable threshold levels. If the color value is identified as being outside an acceptable range, then the system identifies particles having this color value as undesirable product and signals for their removal from the process stream. The rejected particles may be removed by correlating their movement along the moving surface with image acquisition such that the location of the rejected particles can be precisely identified and their removal effectuated by physical means.
An analogous automated system which is capable of sorting scrap metal particles based upon their color has been disclosed by U.S. Pat. No. 5,676,256 to Kumar, et al. (“Kumar”). Yet another approach to automated inspection is provided by U.S. Pat. No. 5,519,225 to Mohr, et al. (“Mohr”) which describes the use of radiographic inspection methods to examine particles in a process stream. Mohr discloses the use of a dual radiation source to alternatively irradiate particles in the process stream with neutrons and X-rays or gamma rays and a dual modality gas ionization detector to detect the radiation after it has passed through the particles. The detected radiation is processed and displayed on a monitor, enabling objects formed with different attenuating materials to be distinguished. The Gigliotti, Kumar, and Mohr patents are incorporated by reference in their entirety as if fully set forth in this specification.
Despite the cost savings and improved sorting capabilities attainable through the use of automated inspecting and sorting systems, there has been considerable resistance to their widespread adoption primarily due to uncertainties associated with the accuracy, reliability and qualification of these systems. For example, the performance of automated inspecting and sorting systems may deviate from the norm due to issues such as nonuniform illumination of the particles, errors occurring during image acquisition and/or mechanical problems during sample transport.
In addition, repeated verification and calibration of automated sorting systems is typically necessary to ensure that the sorting process is functioning properly. These processes can result in significant down time since they typically require that the process flow be stopped so that calibration procedures may be performed without inadvertently allowing undesirable particles to pass through the system and be introduced into the manufactured article. There is therefore a continuing need for the development of automated inspecting and sorting systems which have improved consistency and reliability, operate at reduced cost and can be accurately verified and calibrated with minimal interruption of the process flow.